Regenerative Medicine for Respiratory Diseases

䡵Regenerative Medicine Regenerative Medicine for Respiratory Diseases JMAJ 47(7): 333–337, 2004 Tatsuo NAKAMURA Associate Professor, Institute for F...
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䡵Regenerative Medicine

Regenerative Medicine for Respiratory Diseases JMAJ 47(7): 333–337, 2004

Tatsuo NAKAMURA Associate Professor, Institute for Frontier Medical Sciences, Kyoto University

Abstract: Tissue engineering is an important technology for supporting regenerative medicine, producing tissues in incubation bottles in a tissue culture laboratory. In addition, a newer approach to regeneration of tissues in the human body, called in situ tissue engineering, has been devised in Japan and is attracting attention as a new means of bringing out our body’s potential. New therapeutic methods for chronic respiratory disorders such as pulmonary emphysema and pulmonary fibrosis, as well as for primary pulmonary hypertension for which there have been no curative treatment measures available, are currently under investigation. This paper provides a full account of the developments. In situ tissue engineering causes regeneration of the lung parenchyma itself within the lungs using various growth factors and collagen, which serves as a scaffold for tissue regeneration. A new type of tracheal prosthesis has been in clinical use since 2002 in which new technique was applied to induce autologous tissue regeneration. Key words: Tissue engineering; Artificial trachea; In situ tissue engineering; Primary pulmonary hypertension

Introduction The number of patients suffering from dyspnea after surgical resection of carcinoma of the lungs and patients with chronic respiratory disorders such as pulmonary emphysema or pulmonary fibrosis is expected to increase continuously in the future. Primary pulmonary hypertension is also a progressive disease, though lower in incidence, with an extremely poor prognosis. This paper reviews the present

state of regenerative medical care for these respiratory disorders, with particular reference to the progress of research aimed at application in clinical settings.

Artificial Trachea (Bronchus) Obstructive disorders and stenosis of the central airway are mainly caused by infiltration of lung cancer or thyroid carcinoma. These disorders are rarely indicated for surgery because

This article is a revised English version of a paper originally published in the Journal of the Japan Medical Association (Vol. 129, No. 3, 2003, pages 369–372).

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firmed over a 5 years observation.3) This tissue regeneration-type tracheal prosthesis is now clinically used as a trial upon an ongoing ethical committee review.

Regeneration of Pulmonary Parenchyma and In Situ Tissue Engineering Fig. 1 Autologous tissue regeneration type tracheal prosthesis A structural equivalent of tracheal cartilage comprising a polypropylene stent spirally wrapped around a Marlex mesh (left) and coated with collagen sponge. Autologous tissue regenerates in this collagen scaffold layer.

of lack of reconstructive procedures, despite an extensive resection of the malignancies. Development of an artificial trachea that can be used safely even in such patients has already begun in early 1940s. Initially, a variety of attempts were made to replace the affected trachea with an artificial tube like a vascular prosthesis, but all proved unsuccessful. Artificial tracheas were then developed using the new idea of having autologous tissue regenerate on the surface of a tracheal prosthesis. This artificial trachea is composed of a Marlex mesh tube reinforced by a spiral stent and is conjugated with collagen that has a good affinity for tissues (Fig. 1). Okumura et al. were the first to succeed in the use of this artificial trachea for regeneration of a 5-cm canine cervical trachea.1) The host’s tissues replace the collagen on the implanted artificial trachea while the mesh and stent integrate as being embedded with the host’s tissues. Further, the epithelium spreads from the stumps, and eventually, continuously covers the luminal surface. The use of a Y-shaped collagen-conjugated prosthesis was demonstrated to be safe and effective in the reconstruction of the carina tracheae, which is the most difficult operation in the surgery of respiratory organs.2) The long-term safety of carinal reconstruction has been con-

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Regeneration of the lungs, unlike the liver, has not been recognized. However, the recent advances in regenerative medicine have shown that the lungs can also be regenerated. Regeneration of destroyed pulmonary parenchyma, if feasible, will enable treatment of disorders for which there has long been no curative treatment, such as pulmonary emphysema, pulmonary fibrosis, and pulmonary hypertension. A group of investigators at Harvard Medical School seeded the lung cells obtained from a sheep that had been trypsinized on biodegradable polyglycolic acid (PGA) fiber mesh and incubated with added growth factors such as epithelial cell growth factor (EGF) and basic fibroblast growth factor (bFGF). The investigators implanted the cells on fiber mesh in the subcutis of nude mice. One week later, the implant was noted to have become a tissue resembling lung tissue capable of producing collagen and surfactants. At 2 weeks after the implantation, an extracellular matrix with an alveolar structure (i.e., type I and type II alveolar epithelium and bronchioid patterns) had formed.4) Thus, even the lungs can be formed if pulmonary cells are used under certain conditions. Apart from such attempts, studies aimed at regeneration of new lung tissues with a scaffold placed in the lungs have been conducted by Itoi et al. in Japan.5) When a collagen sponge, which serves as a scaffold for regeneration, is implanted in the lung of a rabbit, the implanted collagen is gradually degraded and absorbed and cells infiltrate it. On Day 5 after the operation, tubular structures had formed in the border of the surrounding lung tissue and had

REGENERATION OF THE LUNG USING IN SITU TISSUE ENGINEERING

Fig. 2

Tissue regenerated in the collagen sponge scaffold implanted in the lung (Courtesy of Dr. S. Itoi, Institute for Frontier Medical Sciences, Kyoto University) Note the regenerated duct cells forming tubular structures continuous to adjacent normal regions. Photomicrograph at 5 weeks after implantation. Normal lung tissue at left. Lung tissue can be regenerated with the new tissue engineering technique.

joined with normal lung tissue (Fig. 2). The changes move in due course to the central portion of the implant, so that the implanted collagen scaffold becomes replaced by newly regenerated lung tissue. The cells showing tubular structures are considered to be of Clara cell or type II alveolar cell origin from the results of the immunostaining examination. The method of regenerating tissue in situ with a scaffold placed in the body is referred to as in situ tissue engineering, and there is growing hope for its clinical application. In addition, development of treatment methods to repair and regenerate lung tissues using growth factors and cells is also in progress as described below.

Pulmonary Fibrosis It is now generally recognized that pulmonary fibrosis arises from progressive lung remodeling caused by proliferating extracel-

lular matrix due to inflammatory cells. The strategy of treatment has been mainly suppression of inflammation, but recently, progress has been made in the areas of tissue repair and regeneration. The pathogenesis of pulmonary fibrosis are implicated in cytokines, such as fibroblast growth factor (FGF) and transforming growth factor (TGF)-␤, which are elaborated by inflammatory cells. Meanwhile, hepatocyte growth factor (HGF) inhibits the effects of these cytokines; intrapulmonary collagen levels are maintained via equilibrium between these factors. Degradation of extracellular matrix is effected by matrix metalloproteinase (MMP) and other proteases, and it has been demonstrated that the tissue level of MMP is elevated in patients with pulmonary fibrosis. In view of this, research in regenerative medicine is currently in progress to regenerate a normal lung from fibrotic pulmonary structure by effective intrapulmonary administration of HGF and other cytokines and MMP.6)

Pulmonary Emphysema The number of the patients of pulmonary emphysema has been rising progressively; the disease is the third or fourth leading cause of death in Western countries. Corticosteroids, expectorants and bronchodilators prescribed for the treatment of pulmonary emphysema are aimed at suppressing the progression of the disease. Pulmonary emphysema is known to progress due to imbalance between proteases and antiproteases in lung tissues. Accordingly, administration of such growth factors as HGF, EGF and TGF-␤, use of protease inhibitors to elastase, MMP and cathepsin, and use of inhibitors to all-trans retinoic acid (ATRA) and other neutrophil activators are being assessed as a drastic therapy.7) Medication with these substances represents an attempt to regenerate pulmonary tissues by stimulating local stem cells that may remain in the

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Alveoli

Alveoli

Primary pulmonary hypertension

Blood vessel

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Blood vessel regeneration

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Regenerated blood vessels

Fig. 3 Shema showing the new treatment method for primary pulmonary hypertension Endothelial progenitor cells (EPC) collected from autologous peripheral blood and grown in vitro are injected directly into the lung to allow neoangiogenesis for improvement of pulmonary hypertension.

damaged lung. Studies in this field had been confined to laboratory investigations using small rodents, therefore, there is a wide gap from the situation in clinical settings. Toba et al. demonstrated regeneration from emphysematous lung tissues with an improvement in pulmonary function by trans-bronchoscopic administration of TGF-␤ in a canine (beagle) model of pulmonary emphysema induced with intra-airway elastase spraying, thus taking a distinct step toward clinical application.8)

Primary Pulmonary Hypertension (PPH) Primary pulmonary hypertension is a disease with an extremely poor prognosis that is characterized by incipient symptoms of shortness of breath on exertion and right cardiac failure and

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an invariably fatal outcome in 2–10 years (average: 5 years). While lung transplantation is used to treat this disease mainly in Western countries, a novel treatment based on regenerative medicine has been devised and is attracting attention. Endothelial progenitor cells (EPC) concerning angiogenesis have been demonstrated to be present in a peripheral blood cell subpopulation called peripheral blood mononuclear cells (PBMCs). Autologous EPC isolated from the peripheral blood of the patient and grown in culture are injected into the lung to regenerate new blood vessels in the lesion for treatment of PPH (Fig. 3). Takahashi et al. trans-bronchoscopically injected autologous EPC that had been isolated from the peripheral blood and grown in vitro into the lung tissue of a canine PPH model. This resulted in regeneration of new blood vessels

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in the lung of the dog, which then showed improvement of cardiac function, which had been depressed due to pulmonary hypertension.9,10) The pioneering results from Japan have shed a great ray of hope for the treatment of PPH, for which lung transplantation has been the only curative treatment available.

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Conclusion With the advent of the twenty-first century, regenerative medical care has been steadily applied in clinical settings to respiratory diseases as well. A new treatment modality is thus being developed even for disorders for which no radical treatment has been available. REFERENCES 1)

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Organs 2000; 23: 718–724. Cortiella, J., Vacanti, C.A. and Vacanti, M.P.: Tissue engineered lung. Tissue Eng 2000; 16: 661. Itoi, S., Nakamura, T. and Shimizu, Y.: In situ tissue engineering for lung regeneration using a collagen sponge scaffold. ASAIO J 2001; 47: 173. Yaekashiwa, M., Nakayama, S., Nukiwa, T. et al.: Simultaneous or delayed administration of hepatocyte growth factor equally represses the fibrotic changes in murine lung injury induced by bleomycin. A morphologic study. Am J Respir Crit Care Med 1997; 156: 1937– 1944. Barnes, P.J.: Novel approaches and targets for treatment of chronic obstructive pulmonary disease. Am J Respir Crit Care Med 1999; 160: S72–S79. Toba, T., Takahashi, M., Fukuda, M. et al.: A study on bronchoscopic preparation of dog models for interstitial pneumonia and emphysematous lungs. Jpn J Chest Surg 2002; 16: 473. (in Japanese) Takahashi, M., Nakamura, T., Toba, T. et al.: New therapeutic option for primary pulmonary hypertension utilizing endotherial progenitor cells. ASAIO J 2002; 48: 188. Takahashi, M., Nakamura, T. and Katou, H.: Transplantation of endothelial progenitor cells into the lung to alleviate pulmonary hypertension in dogs. Tissue Engineering in press.

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